Various types of curable polymer compositions are known. Generally, such compositions include relatively high molecular weight compounds which, upon curing, form an integrated network or structure. The curing may be by a variety of means, including: through use of chemical curing agents; use of catalysts; and, by irradiation. The final physical properties of the cured material are a function of a variety of factors, most notably: number and weight average polymer molecular weights; cross-link density; the structure of the polymer backbone; and, the nature of additives or adjuvants etc. The term "cured", as used herein, means cross-linked or chemically transformed to a set or relatively insoluble condition.
The present invention particularly concerns binders to hold magnetically susceptible or magnetizable particles in a matrix and to adhere them to a substrate of a magnetic recording medium. For example, the present invention concerns binder compositions usable for audio tape, video tape, computer disks or the like. While it is foreseen that binder compositions according to the present invention may be used for other purposes, and in other industries, they are particularly well-suited for application to the magnetic recording media industry.
Generally, a magnetic recording medium comprises a substrate, in the form of a tape or a disk, on which is coated a magnetic recording layer comprising a polymeric binder composition having magnetically susceptible pigment dispersed therein. The binder/pigment composition is coated on the substrate to a desired thickness, and is then dried and cured to bind the pigment to the substrate.
The magnetic pigment, which stores video, audio or other information, generally comprises needle-like particles, typically less than one micron in length. Commonly used pigments include gamma iron oxides, doped iron oxides, and chromium dioxide. The pigment loading, i.e. the weight or volume percent of pigment in the coated composition, is generally high, since recording "bit" density is important. Generally, oxide loadings range from about 55 to 90% of the weight of a solid cured film.
A variety of substances are used for the substrate, including aluminum disks and flexible polyester films. Flexible substrates may be provided in a variety of forms, including tapes, disks, etc.
Cross-linkable polyurethanes are a particularly useful family of resins usable in binder compositions. Reasons for this include the fact that cured polyurethanes generally exhibit good abrasion resistance. That is, they are relatively tough, and do not rapidly wear off, for example when being rubbed across a recording head or the like. Further, they generally operate well over a relatively wide temperature range and, thus, will withstand storage in cold warehouses, and will also operate well on hot equipment. Polyurethanes are also readily available, easily handled, and their chemical and physical properties are fairly well known and widely studied.
In a typical manufacturing process, a dispersion is prepared of the uncured polymer resin and a magnetic pigment, in a readily evaporated solvent such as tetrahydrofuran, cyclohexanone, methyl ethyl ketone, toluene, or methyl isobutyl ketone. The dispersion is then coated onto the substrate, dried and cured. In many instances the dispersion includes adjuvants, such as lubricants, wetting agents or the like, therein to facilitate manufacture and performance of the final product.
In many conventional applications the polyurethane polymer is thermally cured. Typically, thermal curing agents comprise multi-functional isocyanate oligomers, which are thought to promote cure via chemical reaction with active hydrogen-containing functional groups (carbamate, --NH groups or hydroxyl groups) in the polyurethane polymers. Typical curing agents are bi- and preferably tri-functional, to generate substantial cross-linking.
There have been a number of problems with thermal cured systems. Many of these problems relate to the following:
1. First, it is a slow chemical process. A substrate having a partially cured magnetic coating thereon must be stored in a curing location for a considerable period of time, often three to seven days.
2. If tape, such as video tape or the like, is involved, then extremely long tapes are curing while rolled up in reel-like form. Under such circumstances, blocking of the tape coatings can occur, in which adhesion increases with time between adjacent layers of tape in the reel. Upon attempting to unwind the tape at some later time, one finds that the blocking phenomenon has damaged the magnetic tape surface irreparably. While in some instances this may be controlled through utilization of appropriate surface lubricants, it is nevertheless a problem and involves added expense and increased care.
3. Long curing ovens may be needed, which use valuable production space and consume large quantities of thermal energy.
4. Many curing agents undergo numerous side reactions. For example, isocyanates react readily with water. Consistency of cure may be difficult to achieve, due to variations in humidity, and other means of contamination with water. A consistent, high quality, binder film, however, is important for good overall product operation.
5. Curing agents begin to react with reactive moieties in the polyurethane resin, as soon as they are placed in a composition or mixture therewith. Thus, some curing can occur before the dispersion is applied to the substrate. Problems with uncontrolled increase in dispersion viscosity with time, include: difficulties in handling the dispersion solution; difficulties in obtaining consistency of cure, and constant film composition and thickness; difficulties in obtaining appropriate orientation of the pigment on the substrate; and, difficulties due to loss of material, should a production run be stopped or delayed for any reason after the resin has begun curing.
6. Another problem with thermal or chemical curing agents is that they typically only generate cross-linking at active hydrogen-containing reactive sites (carbamate or hydroxyl groups) in the polyurethane. Thus, cross-link density is limited by the presence of these active functional groups. Some preferred polyurethanes include relatively few carbamate and hydroxyl moieties therein, per unit weight. The amount of cross-linking for such polyurethanes, even using high concentrations of curing agents, would be relatively low. Greater cross-link density may be desirable if a relatively strong binder film is desired.
7. In binder coatings such as are used in magnetic recording media, it is desirable to eliminate any non-cross-linked polymer. Non-cross-linked polymer, which remains extractable when the polymer film is immersed in a solvent, can migrate or "bloom" to the tape surface, and can interfere with recording heads or the like. It is desirable, from experience, to maintain the percent by weight extractable polymer at less than about 3 wt.%. In thermally cured systems, i.e. systems cross-linked by means of isocyanate or the like, a substantial amount of curing agent may be necessary in order to achieve less than 3 wt.% extractable polymer. This adds cost, since a substantial amount of curing agent may be necessary. Further, control of side reactions, especially with such a high amount of curing agent, may be a problem as indicated above.
In efforts to avoid thermal curing agents, resin compositions usable as binders for magnetic recording media have been developed which are cured upon exposure to ionizing radiation. Polyurethane compositions have been developed which are cured through a variety of free radical reactions initiated by various types of irradiation. A useful method of irradiation is electron-beam irradiation, since with electron beam (EB) irradiation a relatively quick, substantially complete cure can be obtained.
Known electron-beam cured polyurethane compositions have not been completely satisfactory, when applied as binding agents for magnetic pigments or the like. For example, if relatively low molecular weight polymer compositions are used in the formulation, the dried coatings (before cure) tend to be tacky and easily damaged (for example by incidental contact with harder surfaces such as idler rolls in the manufacturing process). Also, cross-link density is still not particularly high, so the cured resin is not very strong.